There was a previous discussion about making `brew linkage --test` fail
for unrequested dependencies (#9172). I'm not sure what the outcome of
that was, but it still seems like a good idea to try to help us find
cases of opportunistic linkage as they happen rather than when they
cause CI failures in another PR sometime later.

Let's do this by adding a `--strict` flag to `brew linkage --test`. My
intention is for `brew linkage --test --strict` failures to be warnings
rather than errors in CI, which should mitigate some of the concerns
about doing this that were raised in #9172.